In a digital camera or a video camera that is one of image pickup apparatuses, a charge coupled device (CCD) image sensor or a complementary metal-oxide semiconductor (CMOS) image sensor is widely used as an image pickup device. In the image pickup device, an image pickup element in which a plurality of photoelectric conversion elements are two-dimensionally arranged with a predetermined distance (hereinafter, referred to as “pixel pitch”) and a color filter array disposed on a light incident surface of the image pickup element are provided. The image pickup device discretely samples object light that has entered the image pickup device through an image pickup lens by the image pickup element and the color filter array, to obtain color image data.
The image pickup device has a resolution limit (Nyquist frequency) defined by a pixel pitch of the image pickup element or a color arrangement pitch of the color filter array. Therefore, when object light that has a high frequency component equal to or higher than the Nyquist frequency enters the image pickup device, the high frequency component equal to or higher than the Nyquist frequency becomes a folded component in a real space, and thus stripe-like pattern (moiré) in which color of the image data and luminance are periodically changed may occur. Accordingly, an optical low pass filter (OLPF) in which the Nyquist frequency is set to a cutoff frequency is typically disposed between the image pickup lens and the image pickup element, and the high frequency component of light entering the image pickup element is removed by the OLPF.
In a camera capable of capturing both a still image and a moving image, a reading pitch is different between in a still image mode and in a moving picture mode, and the Nyquist frequency is accordingly different therebetween. In the moving picture mode, normally, the reading pitch is larger than the reading pitch in the still image mode, and the Nyquist frequency is smaller than the Nyquist frequency in the still image mode. Therefore, in the moving picture mode, moiré is more easily generated from a low frequency region than in the still image mode. However, in the past, a high priority is given to resolution of the still image, and the cutoff frequency of the OLPF is set to the Nyquist frequency in the still image mode. Thus, an OLPF optimal to both modes is not provided.
There is proposed that an OLPF having an optimal cutoff frequency is provided for each mode, and the OLPF is switched depending on the modes (see PTL1). Moreover, there is provided that, in an OLPF in which a TN liquid crystal is sandwiched between, a pair of liquid crystal plates each having birefringence, the TN liquid crystal is driven to be turned on or off to change a ps separation width of transmitted light of the OLPF into two modes (see PTLs 2 and 3). Note that changing the ps separation width makes it possible to change the cutoff frequency.